System and method for detecting, correcting and measuring depth movement of target tissue in a laser surgical system

Abstract

A system and method for detecting, measuring and correcting for movements of a target in a medical analytic or surgical system utilizes generally the principles of confocal microscopy. A pinhole and photodetector combination is positioned behind optics of a system for delivering an ophthalmic surgery laser beam, for example. As in a confocal microscope, the optics are arranged such that a beam waist is formed precisely at the pinhole when the target is in its nominal position. When the target moves from its nominal position in the depth direction, the signal from the pinhole/photodetector combination decreases. The change in the signal can be used to drive the objective lens of the optics so as to move with the moving target. Alternatively, the signal can be used to drive the pinhole/photodetector assembly so as to again attain peak signal, in this way allowing the target's shift to be measured.

I claim: 
     
       1. An optical system for detecting and correcting for movements in a depth direction of a target at which a treatment laser beam is directed, the treatment laser beam passing through a common objective lens with the optical system, comprising, objective lens means at the front of the optical system, to be positioned adjacent to a target lying on an optical axis of the objective lens means and genteelly at the focus of the objective lens means,   illumination means for sending an illuminating light beam toward the target through the objective lens means, to an illuminating beam focus at the position of a reflective surface associated with the target when the target is in a nominal position,   optical means behind the objective lens means for receiving light reflected from the reflective surface associated with the target and passed through the objective lens means and for focussing the reflected light to a rear focus or beam waist,   a pinhole structure with a pinhole located at the beam waist when the target is in a nominal position with the reflective surface at the illuminating beam focus,   photodetector means behind the pinhole structure and positioned to receive the illuminating beam as reflected from the reflective surface of the target and passed through the objective lens means, the optical means and the pinhole, the photodetector means including means for measuring the intensity of light received through the pinhole,   treatment laser means for producing a treatment laser beam, with means for folding the treatment laser beam into the optical system so as to pass the treatment laser beam through the objective lens means toward a treatment laser focus at the target, with other optical means for steering the treatment laser beam independently of the illuminating light beam, and   target following means connected to the photodetector means and including driving means for moving the objective lens means outwardly toward the target or inwardly away from the target so as to maintain essentially constant focus of the illuminating light beam on the target, the target following means including feedback means responsive to a reduction in light intensity at the photodetector means, for moving the objective lens means until the light level sensed at the photodetector means is maximized, indicating the location of the beam waist at the pinhole and the location of the reflective surface at the focus of the illuminating light beam, whereby the focus of the treatment laser beam is properly relocated relative to the target as desired.   
     
     
       2. A system according to claim 1, wherein the reflective surface associated with the target is a tear layer of the cornea of a human eye. 
     
     
       3. A method for detecting and correcting for movements in a depth direction of a target at which a treatment laser beam is directed, comprising, directing an illuminating light beam from an illuminating light source toward the target through an objective lens means, to a focus at the position of a reflective surface associated with the target,   providing a pinhole structure with a pinhole, in an optical system behind the objective lens means,   reflecting the illuminating light beam off the reflective surface associated with the target, and receiving the reflected light through the objective lens means and through the optical system to a rear focus or beam waist of the reflected light at the pinhole when the target is in a nominal position with the reflective surface at the focus of the illuminating light beam,   directing a treatment laser beam through the objective lens means to a treatment laser focus at the target, which may be a different focus from the focus of the illuminating light beam but in a known relationship thereto, the treatment laser beam having optics enabling the independent manipulation of its focus,   detecting the level of light passed through the pinhole with a photodetector means positioned behind the pinhole, and   operating a feedback loop to move the objective lens means outwardly toward the target or inwardly away from the target in response to a reduction in light intensity sensed by the photodetector means behind the pinhole, until the light level sensed at the photodetector means is maximized thereby indicating the location of the beam waist at the pinhole and the location of the reflective surface at the focus of the illuminating light beam, and thereby refocussing the treatment laser beam at the proper depth at the target, thereby maintaining essentially constant focus on the target.   
     
     
       4. The method of claim 3, wherein the reflective surface associated with the target is a tear layer of the cornea of a human eye, and wherein the treatment laser beam is of such power and repetition rate as to be capable of therapeutic treatment inside the eye. 
     
     
       5. The method of claim 3, wherein the movement of the objective lens means to maximize sensed light level is accomplished by monitoring an intensity signal from the photodetector means in a microprocessor, and automatically directing a motor means with the microprocessor to move the objective lens means in a way as to maximize sensed light intensity after a decrease in light level is detected. 
     
     
       6. A method for detecting and correcting for movements in a depth direction of a target at which a treatment laser beam is directed, comprising; directing an illuminating light beam from an illuminating light source toward the target through an objective lens means, to a focus at the position of a reflective surface associated with the target,   providing a pinhole structure with a pinhole, in an optical system behind the objective lens means,   reflecting the illuminating light beam off the reflective surface associated with the target, and receiving the reflected light through the objective lens means and through the optical system to a rear focus or beam waist of the reflected light at the pinhole when the target is in a nominal position with the reflective surface at the focus of the illuminating light beam,   directing a treatment laser beam through the objective lens means to a treatment laser focus at the target, which may be a different focus from the focus of the illuminating light beam but in a known relationship thereto.   detecting the level of light passed through the pinhole with a photodetector means positioned behind the pinhole, and   dithering the pinhole and photodetector assembly in and out in order to determine which direction of travel results in an increased light level at the photodetector when the target has moved, and then moving the objective lens means in the appropriate direction as determined from the dithering until the sensed light level at the photodetector is maximized, equivalent to obtaining a balanced dither signal, thereby indicating the location of the reflective surface at the focus of the illuminating light beam, and thereby refocussing the treatment laser beam at the proper depth at the target.